Torque-based catheter articulation

ABSTRACT

A robotic surgical system configured for the articulation of a catheter includes an input device, a control computer, and an instrument driver having at least one motor for displacing the pull-wire of a steerable catheter wherein the control computer is configured to determine the desired motor torque or tension of the pull-wire of a catheter based on user manipulation of the input device. The control computer is configured to output the desired motor torque or tension of the pull-wire to the instrument driver, whereby at least one motor of the instrument driver implements the desired motor torque to cause the desired pull-wire tension to articulate the distal tip of the catheter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/867,980, entitled “TORQUE-BASED CATHETER ARTICULATION,” filed on Sep.28, 2015, and published as U.S. Patent Application Publication No.2016/0015937, which is a continuation of U.S. patent application Ser.No. 13/828,342, entitled “TORQUE-BASED CATHETER ARTICULATION,” filed onMar. 14, 2013, and issued on Nov. 3, 2015 as U.S. Pat. No. 9,173,713.The disclosure of the above-referenced patent applications is herebyincorporated by reference in their entirety herein.

FIELD OF THE PRESENT DISCLOSURE

The present disclosure generally relates to robotic surgical systems forperforming minimally invasive diagnostic therapeutic procedures andparticularly to robotic catheter systems for steerable catheters.

BACKGROUND

Robotic surgical systems and devices are well suited for use inperforming minimally invasive medical procedures, as opposed toconventional techniques that may require large incisions to open thepatient's body cavity to provide the surgeon with access to internalorgans. For example, a robotic surgical system to be utilized tofacilitate imaging, diagnosis, and treatment of tissues which may liedeep within a patient, and which may be preferably accessed only vianaturally-occurring pathways such as blood vessels or thegastrointestinal tract.

One such robotic surgical system that may be utilized in a minimallyinvasive procedure is a robotic catheter system. A robotic cathetersystem utilizes a robot, external to the patient's body cavity, toinsert a catheter through a small incision in a patient's body cavityand guide the catheter to a location of interest. Catheters may besteerable for movement in multiple axes including axialinsertion/retraction, axial rotation, and deflection/articulation, whichencompasses radial bending in multiple directions. To accomplishsteering, one or more pull-wires are attached to the distal end of anarticulating section of a catheter and extend the length of thecatheter. The distal tip of a catheter may then be controlled via thepull-wires, i.e., by selectively operating tensioning control elementswithin the catheter instrument.

Kinematic modeling is utilized to predict catheter tip movement withinthe patient anatomy. The amount of displacement of a pull-wire isgenerally proportional to the amount of articulation. At times, thecalculated motion of the catheter does not precisely match the actualmotion within the patient's anatomy. Various elements can affect theamount of articulation for a given pull-wire actuation, including thepresence of unanticipated or un-modeled constraints imposed by thepatient's anatomy, particularly given the tortuous path that thecatheter must traverse. Minimization of differences between actual andpredicted kinematic functions is desirable to achieve a highlycontrollable robotic surgical system.

SUMMARY

A robotic surgical system may include at least one input deviceconfigured to output desired catheter positioning information; a controlsystem operatively connected to the input device and configured toreceive the positioning information from the input device and totranslate the positioning information into at least one output motortorque command; at least one instrument driver operatively connected tothe control system and responsive to the output motor torque command toarticulate the distal portion of a catheter instrument,

The control system of the robotic surgical system may include analgorithm in which at least one set of instructions defines a cathetermovement profile using motor torque as an output to the instrumentdriver. The instrument driver of the robotic surgical system may alsoinclude at least one torque measuring device. The robotic surgicalsystem may use closed loop feedback to sense any difference betweenactual motor torque and the output motor torque so as to adapt theelectrical current to correct for any difference with the output motortorque and/or adapt kinematic parameters to determine adjustments to theoutput motor torque. A motor servo may use torque data from the torquemeasuring device to adjust the electrical current supplied to a motor.The control system may also use the torque data to adjust the operationparameters.

A method of articulating a catheter instrument in a robotic surgicaldevice, comprising the steps of inserting a catheter instrument into ananatomical lumen of a patient; manipulating an input device to generatepositioning information for a desired position for a distal portion ofthe catheter instrument which is steerable; communicated to a controlsystem; converting the positioning information from the input deviceinto at least one output motor torque; and communicating the outputmotor torque to an instrument driver thereby causing the instrumentdrive to articulate the distal end portion of the catheter instrument.The method may further include the step of measuring current motortorque. The method may further include the step of adjusting theelectrical current of at least one motor in response to an error signalthat current motor torque is different from the output motor torque.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary robotic surgical system.

FIG. 2 is an illustration of an exemplary catheter assembly of thesurgical system of FIG. 1.

FIG. 3 is a schematic showing a kinematic relationship between pull-wiredisplacement and catheter tip articulation.

FIGS. 4 and 5 are partially exploded views of the catheter assembly ofFIG. 2.

FIG. 6 illustrates an exemplary steerable catheter with pull-wires.

FIG. 7 is a model of a catheter assembly as a system of springs.

FIG. 8 is a flow diagram for catheter steering using desired motorposition as an output to motor position servo control of the instrumentdriver.

FIG. 9 is a flow diagram for catheter steering using desired motortorque as the output to the instrument driver.

FIG. 10 is a model-based force control block diagram, which is oneimplementation of the embodiment shown in the flow diagram of FIG. 9.

FIGS. 11A and 11B illustrate an exemplary torque sensor.

FIGS. 12A and 12B and 13A and 13B illustrate an exemplary benefit of thepresent disclosure in the articulation of a steerable catheter.

FIGS. 14 and 15 illustrate an exemplary benefit of the presentdisclosure in the articulation of a steerable catheter.

FIG. 16 is a flow diagram illustrating the use of tension information toequalize or eliminate tension in pull-wires.

DETAILED DESCRIPTION

Referring now to the discussion that follows and also to the drawings,illustrative approaches to the disclosed assemblies are shown in detail.Although the drawings represent some possible approaches, the drawingsare not necessarily to scale and certain features may be exaggerated,removed, or partially sectioned to better illustrate and explain thepresent disclosure. Further, the descriptions set forth herein are notintended to be exhaustive or otherwise limit or restrict the claims tothe precise forms and configurations shown in the drawings and disclosedin the following detailed description.

Referring to FIG. 1, a robotic surgical system 100 is illustrated inwhich an apparatus, a system, and/or method may be implemented accordingto various exemplary illustrations. System 100 may include a roboticcatheter assembly 102 having a sheath instrument 104 and/or a catheterinstrument 106. Catheter assembly 102 is controllable using a roboticinstrument driver 108 (generally referred to as “instrument driver”).During use, a patient is positioned on an operating table or surgicalbed 110 to which robotic instrument driver 108 is coupled or mounted. Inthe illustrated example, system 100 includes an operator workstation112, an electronics rack 114 including a control computer (not shown), asetup joint mounting brace 116, and instrument driver 108. A surgeon isseated at operator workstation 112 and can monitor the surgicalprocedure, patient vitals, and control one or more catheter devices.

Operator workstation 112 may include a computer monitor to display athree dimensional object, such as a catheter displayed within orrelative to a three dimensional space, such as a body cavity or organ,e.g., a chamber of a patient's heart. In one example, an operator usesone or more input devices 120 to control the position of a catheter orother elongate instrument. In response to actuation of the input deviceby a user, the input device can output positioning information for thedesired position of the catheter instrument, including thethree-dimensional spatial position of the distal end of a steerablecatheter. System components, including the operator workstation,electronics rack and the instrument driver, may be coupled together viaa plurality of cables or other suitable connectors 118 to provide fordata communication, or one or more components may be equipped withwireless communication components to reduce or eliminate cables 118.Communication between components may also be implemented over a networkor over the internet. In this manner, a surgeon or other operator maycontrol a surgical instrument while located away from or remotely fromradiation sources. Because of the option for wireless or networkedoperation, the surgeon may even be located remotely from the patient ina different room or building.

Referring now to FIG. 2, motors within instrument driver 108 arecontrolled such that carriages coupled to mounting plates 204, 206 aredriven forwards and backwards on bearings. As a result, a catheter canbe controllably manipulated while inserted into the patient. Instrumentdriver 108 contains motors that may be activated to control bending ofthe catheter as well as the orientation of the distal tips thereof,including tools mounted at the distal tip.

The articulation of catheters is normally performed by actuatingpull-wires that extend the length of the catheter and are attached tothe distal end of an articulating section of a catheter. In order toarticulate the catheter, the pull-wire is displaced at the proximal endto articulate the distal end of the catheter. Typically, the amount thatan articulating section of a catheter articulates is determined bycalculating the change in path length that an actuating pull-wire takes.For a straight catheter, that length is equal to the articulatingsection, L_(o). As the catheter bends (where α is the angle from theneutral axis, r_(c) is the radius of the catheter, and τ is thearticulation angle), the path length is equal toL_(o)−cos(τ/90)*r_(c)*τ. The difference—(α/90)*r_(c)*τ—is the distancethe pull-wire must be actuated to make a catheter articulate to an angleτ, as illustrated in FIG. 3. From this concept, further solid mechanicand kinematic modeling is used via algorithms in the control computer toconvert a desired catheter position or orientation as provided by theuser into commands to the instrument driver to rotate motors designatedfor each pull-wire.

When a catheter is prepared for use with an instrument, its splayer ismounted onto its appropriate interface plate. In this case, as shown inFIG. 4, sheath splayer 308 is placed onto sheath interface plate 206 anda guide splayer 306 is placed onto guide interface plate 204. In theillustrated example, each interface plate 204, 206 has respectively fouropenings 310, 312 that are designed to receive corresponding driveshafts 314, 316 (FIG. 5 illustrates an underside perspective view ofshafts 314, 316) attached to and extending from the pulley assemblies ofthe splayers 308, 306). Drive shafts 314, 316 are each coupled to arespective motor within instrument driver 108.

Embodiments with less or more than four pull-wires are contemplated bythe present disclosure. When, e.g., a four-wire catheter 304 is coupledto instrument driver 108, each drive shaft 316 thereof is therebycoupled to a respective wire 504-510 (see FIG. 6). As such, a distal end512 of catheter 304 can be articulated and steered by selectivelytightening and loosening pull-wires 504-510. Typically, the amount ofloosening and tightening is slight, relative to the overall length ofcatheter 304. That is, each wire 504-510 typically need not be tightenedor loosened more than perhaps a few centimeters. As such, the motorsthat tighten/loosen each wire typically do not rotate more than, forexample, ¾ of a rotation. Thus, given the solid mechanics and kinematicsof directing the instrument driver, a catheter (or other shapeableinstrument) may be controlled in an open-loop manner, in which the shapeconfiguration command comes in to the beam mechanics and is translatedto beam moments and forces, then translated into pull-wire tensions asan intermediate value before finally translated into pull-wiredisplacement given the entire deformed geometry. Based on the pull-wiredisplacement command, a motor servo can apply the appropriate electricalcurrent to produce the amount of rotation required to displace thepull-wire.

Robotic systems use algorithms to determine the displacement of thepull-wires to achieve the desired articulation of a catheter. However,differences between predicted and actual catheter position can resultfrom the reliance by the kinematic model on certain assumptions and thelack of certain information. With rigid kinematics, simple geometry canbe used to predict the location of any point along the rigid objectgiven the following information: (1) a reference coordinate system; (2)an origin, or point in any coordinate system attached to the object; and(3) an orientation in any coordinate system attached to the object. Evenwith rigid structures, external forces, even gravity, may disrupt theability to solve the location equation given the information above. Ifthe above information is not sufficient to accurately describe theposition of one point of an object from another point on the sameobject, then additional information must be provided, like the weight ofthe object, the forces acting on the object, the strength of the object,etc.

Standard equations and constants, like Poisons ratio, Hertzian stresses,Modulus of Elasticity, and linear stress/strain equations can improve onthe kinematic model but these methods break down once the strains exceedthe standard elastic range (usually about 3%). For example, a slim barmay be straight under no distal loading and the equations to predict thelocation of the distal end are fairly effective. However, when a load isplaced on the beam, the distal end will deflect, or strain under theload. Even in a purely elastic response to the load, the location ororientation of the distal end of the beam is impossible to predictwithout knowing the magnitude, the direction, and the location of theexternal load. Similarly, flexible instruments such as catheters withlow strength can be deflected by unknown loads at unknown locations andin unknown directions. Yet, prediction of the location and orientationof the distal end of a catheter is an important aspect of a roboticcatheter system. The orientation of the distal end of the catheter basedon information measured at the proximal end can better be determinedthrough embodiments of the present disclosure.

To enhance the transfer function between inputs and outputs, anembodiment of the present disclosure determines the output motortorque(s) corresponding to the desired tension in the pull-wires and theinstrument driver 108 acts on the output motor torque command toarticulate the catheter. A kinematic model for catheter articulation cantranslate positional data from the input device 120 at the workstation112 into pull-wire displacement commands whereby the motor(s) of theinstrument driver 108 are rotated in accord with that amount ofdisplacement. In contrast, in an exemplary embodiment of the presentdisclosure, the positional data from input device 120 is translated intopull-wire tension values tied to motor torque, whereby the motors arerotated in accord with the value of the motor torque.

From a user standpoint, the system behaves much the same way in that theuser provides desired catheter motions and the catheter follows. Thedifference is that by using desired pull-wire tension instead of desiredpull-wire displacement, a number of advantages can be achieved. Oneadvantage is the reduction in the number of assumptions in the kinematicmodel to increase accuracy. For example, with pull-wire displacementcalculations, various non-rigid elements affect the amount ofarticulation for a given pull-wire actuation, including (1) pull-wirestiffness; (2) axial catheter stiffness and (3) actual cathetergeometry, including the resultant stiffness of the bending or twistedsection. The displacement of the pull-wire at the actuated or proximalend of the catheter is thus not equal to the displacement at the tip ofthe catheter, but rather a function of the various spring rates. Forinstance, a small pull-wire will stretch more per unit force andtransmit less displacement to the distal tip than a stiffer one thoughboth will transmit the same force.

Catheter movement may be predicted by modeling the catheter assembly asa system of springs. For example, if the following parameters are known:

K_(S): Spring rate of catheter shaft (lbs/in)

K_(T): Torsional Spring rate of bending section (in*lbs/radian)

K_(W): Spring rate of pull-wires (lbs/in)

Further, K_(T) is directly proportional to the Bending Stiffness(lb*in²), where K_(T)*L=K_(B) where L=Length of bending section. Thenthe entire catheter can be modeled as a spring system, as shown in FIG.7. The use of pull-wire tension as the output parameter eliminates theneed to make such assumptions. That is, the displacement at the tip canbe calculated by solving the force equations instead of solving thedisplacement equation. The transfer function from input to output canthus be improved by commanding the instrument driver 108 with torquecommands correlated to the desired output tensions for the one or morepull-wires in the catheter.

Further, in an exemplary variation of that embodiment of the presentdisclosure, the high-level catheter control algorithm can be combinedwith a lower level motor torque control algorithm to accurately drive acatheter in response to a user's commands. Control algorithms may besplit into several parts. One aspect of catheter control, as shown inFIGS. 8 and 9 in block A, may be a high-level open-loop algorithmcombining catheter solid mechanics and kinematics to convert a desiredcatheter position or orientation as provided by the user into motorcommands. Another aspect of catheter control, shown in FIGS. 8 and 9 inblock B, may be a lower-level closed-loop motor servo controller thattakes the motor commands produced by the catheter control algorithms andconverts them to the motor current needed at every time step to achievethe desired catheter motion. FIG. 9 is an exemplary embodiment of thepresent disclosure illustrating the flow diagram for catheter steeringusing desired motor torque as the output to the instrument driver.Though a surgical system may control the catheter through the use ofmotor commands to the motor servos in the form of position commands,increased driving performance can be achieved by altering thearchitecture such that the motor command transferred to the motor servocontroller is a desired motor torque.

Desired motor torques can be determined based on catheter solidmechanics and kinematics control algorithms. For example, the value ofthe desired tension of the catheter pull-wires can be used to determinethe desired motor position. The desired pull-wire tension can betranslated to a desired motor torque by multiplying the pull-wiretension by the radius of the pulleys in the splayers.MotorTorque_(Desired)=PullWireTension_(Desired)*PulleyRadiusOnce the desired motor torque is determined, the instrument driver 108can apply the desired torques to the motors which in turn applies thedesired tension to the pull-wires.

A model-based force control algorithm is an exemplary variation of anembodiment, having the advantages of extreme robustness and stableinteraction with the possible passive environments seen at the outputshaft of the motor. In the case of the articulation axes, eacharticulation motor may be treated as a single degree of freedom torquesource, in which case for this embodiment, the model-based force controlalgorithm collapses into an integral controller on force error combinedwith direct feed-forward of the desired force and a viscous dampingterm.

FIG. 10 is a control block diagram modeling aspects of the flow diagramof FIG. 9 using model-based force control. Since the motor velocity andoutput torque are a function of both motor dynamics as well as the loadproduced by the pull-wire, the motor is not modeled as a simple transferfunction given the inherently non-linear and time dependent pull-wiredynamics that are constantly changing depending on what the catheter istouching, what shape the catheter has, and how the catheter is beingdriven. Instead, the combined motor/catheter pull-wire system is modeledas a motor block which takes as an input the net torques applied to themotor (τ_(elec)-τ_(wire)) where τ_(elec) is the electrical torqueprovided by the amplifiers and calculated by the control algorithm andτ_(wire) is the reaction torque provided by the pull-wire. These torquescombine with the motor dynamics to produce a motor velocity dO/dt whichis then fed into the pull-wire dynamics to produce the torque applied tothe pull-wire.

The benefits of the model-based force control algorithm arise in partfrom its stability for any passive pull-wire load dynamics regardless ofnon-linearity or complicated geometry. In this variation of oneembodiment of the present disclosure, the model-based control algorithmdoes not allow the force controller to attempt to reject any of themotor's inertial forces as a disturbance in order to achieve a highlevel of stability. That is, the force control algorithm does notattempt to make the motor feel like it has less mass than it actuallyhas. For this algorithm, the ratio K_(D)/K_(l) must be greater than theinertia of the motor. Once implemented, the model-based controlalgorithm essentially works by rejecting any friction in the motor (orgearing) such that the resulting closed-loop system feels like an idealfrictionless motor with inertia K_(D)/K_(l). At steady-state and with noacceleration, the integral term in the controller ensures completefriction rejection such that τ_(wire)=τdes. At all other times, thedifference between τ_(wire) and τdes is mostly the inertial force of themotor.τ_(des)−τ_(wire) ≈Jα where α is the motor acceleration.The above approximation becomes more exact as control terms K_(D) andK_(l) become larger. Nevertheless, like any control algorithm,limitations exist on the size of the control gains without excitingother unmodeled dynamics in the system and driving the system toinstability. The model-based force control strategy provides increasedperformance while still meeting robustness requirements and withoutreducing safety.

The model-based force control strategy is one exemplary implementationof the present disclosure. The torque-based strategy of the pull-wiretension control paradigm can be implemented with numerous possible forcecontrol algorithms could be used to get the motors to behave as desired.Further, multiple modifications can be added to the catheter controlkinematics and solid mechanics to fine tune catheter drivingperformance. Exemplary modifications to the motor torque control servoalgorithm include: (1) using additional control strategies to modifyτ_(des) before sending to the motor servo controller; such as, addingadditional damping for enhanced stability or simply for achieving moredesirable dynamics; and (2) replacing the integral controller with ahigh gain, low pass filter to avoid problems (such as integral drift)that typically arise when using integral controllers.

In order to sense the tension in the catheter control wires, a torquemeasuring device can be utilized. One exemplary embodiment of thepresent disclosure, shown in FIGS. 5, 9, 11A and 11B, incorporatestorque sensors 903 installed on the drive shafts 314, 316 that transmitmotion from the motors to the pull-wires. A torque sensor 903 could bealternatively located in other areas of the system. An alternativemethod to measure torque may include a direct measurement of force onthe pull-wires using a force sensor 901. An exemplary variation of thatalternative method may involve using the pull-wire as a strain gauge902.

In one embodiment, the tension in the pull-wire can be calculated simplyby dividing the torque by the effective radius (rdrum+r wire) of thecatheter control wire. Any difference between the actual and commandedmotor torque value (an “error signal”) can be used to make electricalcurrent adjustments by a motor servo or the control computer to reduceor eliminate the error. Thus, knowledge of the tension (or force) beingapplied to the pull-wires permits close loop controlled of the cathetermovement. Further, with the tension information available, controlalgorithms can improve the performance and control of the catheter. Insome instances, the tension information can provide information aboutforces being applied to the catheter and can be fed back to the userhaptically.

Indeed, using tension as a predictor of catheter angulations hasnumerous advantages beyond those discussed above. For example, when theshape of the catheter shaft is unknown as a result of anatomicalconstraints when inserted into vasculature, un-modeled shaft dynamicsmay result in unintended articulation. FIG. 12A shows a catheter with astraight shaft and control wires in positions x₁ and y₁; FIG. 12B showsthe same catheter with the shaft bent—with the tip of the catheterbending to the right when the shaft bends to the right. Even though thepull-wires wires are held in their original positions for bothconditions, namely x₁ and y₁, the tip of the catheter is bent to a newposition. Because the shaft is no longer straight, the distal tip of thecatheter is forced to bend in order to keep the total length between theproximal points x₁ and y₁ and the distal points x₂ and y₂ identical.

The use of a desired motor torque as the output instead of a desiredmotor position improves this issue. Turning to FIGS. 13A and 13B, theinitial conditions are shown to be the same but the pull-wires are heldunder a constant tension. Viewing the kinematic model as weights hungfrom each wire, FIG. 13B illustrates the shaft bent in a similar manneras FIG. 12B, but the distal section remains straight as the path lengthsof the pull wires need not be made identical as the constant tension inthe control wires kept the distal section articulated to the same angle.The position of the pull-wire has changed to x₃ and y₃ to accommodatethe bend in the shaft.

The present disclosure can also enable distal disturbance detection,i.e., when the distal tip of the catheter has been subjected to anunknown force in an unknown direction, often from tissue contact.Without the distal disturbance, a repeatable relationship exists betweencatheter pull-wire tension and position, but the disturbance changes thedisplacement and/or orientation of the tip. Because a difference betweencommanded and actual positions of the catheter will exist as a result ofthe disturbance, the length of the path of the control wires will bedifferent than expected. These changes cause a difference in the tensionof the wires, provided they are moved away from the commanded location.For instance, if the catheter is commanded to articulate to 180° but adistal disturbance prevents the catheter from bending past 90°, then thetension in the inner wires will be higher than expected and the tensionin the outer fibers will be lower due to the difference in path lengths.

FIGS. 14 and 15 illustrate a distal disturbance. The amount of pull-wiremotion can be correlated to motion expected at the distal section byusing a spring model. Using motor torque to steer a catheter, the changein spring rate caused by a disturbance at the distal end of the cathetercan be observed. When a disturbance at distal end prevents the cathetertip from achieving the commanded angulation, that resistance of motionat the distal section will effectively change the spring rate of thecatheter shaft and the equations will fail to predict the motion. Inthis example, the angulation is commanded by moving the control wirefrom position x₁ to x₂. As governed by the equation, this change inspring rate can be determined via knowledge of the tension in thecontrol wires. Thus, tension or motor torque data can be used todetermine adjustments to the motor commands to achieve the desiredtension and adjusting motor commands based on real time tension data.This information can also be fed back into the controls software and canbe presented to the user as haptic, visual or other feedback to relaythe effect of the disturbance.

The distal disturbance shown in FIG. 15 is caused by the catheter comingin contact with an external surface. An alternative distal disturbancemay occur when a therapeutic device such as a stent, an atherectomydevice or a balloon is being advanced through the catheter. Thestiffness of this device cannot be previously modeled because the doctorcan choose from a large variety and size range of devices. But a systemwith tension control can be used to ensure the angle of the tip of thecatheter does not change. The increased spring rate of the catheter asthe therapeutic device is being advanced can be detected by the tensionsensors. Then, this tension or motor torque data can be used todetermine adjustments to the motor commands to maintain the catheter tipposition. This information can also be fed back into the controlssoftware and can be presented to the user as haptic, visual or otherfeedback to relay the effect of the increased stiffness.

Embodiments of the present disclosure also have the advantage ofenabling compensation for time dependent variables in like plasticcreep, which changes the operational working length of the catheter butnot the wires, causing tension to reduce over time. Creep compensationcan also be compensated for if the tension in the control wires isknown. Catheters are generally made from plastic laminates; plastics areknown to change dimensions when subjected to external forces over time.This change over time is known as creep. However, with the presentdisclosure, if the tension in all the wires is less than expected, acatheter shaft length can be compensated for by returning the tensionsto their expected values.

Further, good control of a catheter can be obtained only if the motionexpected to be transmitted to the controls wires are indeed transmitted.For a variety of reasons, motion intended to be transmitted to thearticulating section may fail. Friction in the gearing or othermechanical aspects of the instrument driver may be problematic. Backlashin the drive transmission can be seen as slack or non-taut pull-wires.The control of the catheter can be improved if the scenario is detectedand compensated for adequately, as shown in the flow chart of FIG. 16,through general re-zeroing of the pull-wire tension. The process of FIG.16 may also be used to accommodate shaft shape compensation when theshaft has been subjected to new unknown forces. The tension on thecontrol wires will be equal and near or equal to zero when the catheterarticulating section is straight. However, if the shaft of the catheterhas been deflected, and the catheter has been commanded to be straight,the tension in the catheter control wires will be non-equal. Thus, theequalization and reduction of tension to or near zero is contemplated bythe present disclosure. Using tension sensing to take up wire slack mayalso be performed when the path length for an inside wire is shorterafter a bend in the catheter, such as when the catheter is goingcontralateral over the iliac bifurcation. Further, another use oftension sensing to take up wire slack may occur for a catheter that hasspiraled. Steerable catheters often spiral when pushed through theanatomy, resulting in a shorter path for one wire.

A pull-wire tension control paradigm can also inherently implement asafety check to ensure safe and reliable behavior. Exemplary variationsof the present disclosure may include enabling tension limits on thecontrol wires to prevent breakage, preventing articulation of thecatheter when tension sensing indicates a distal disturbance frompotential tissue contact (as discussed further above) and/or limitingthe combined loading on the catheter shaft to prevent buckling.Specifically, the direct measurement of torque can signal if the tensionin the pull-wire is approaching maximum limits. The catheter controlswires are made of steel and inherently have a breaking strength. Onefailure mode of catheters is when the pull wires break. This can beprevented by monitoring the tension in the cables and preventing themfrom being overstressed.

Further, the tension sensors can be monitored by comparing the motorcurrent at any given moment with the tension measurements. Since thecurrent is related to the torque applied by the motors, a discontinuitybetween the sensor readings and the current could indicate a potentiallyfaulty tension sensor. Similarly, the present disclosure assists inshaft buckling prevention. The tensile loads in the catheter controlwires are reacted by a compressive load in the catheter shaft. Like thewires in tension, the shaft has a loading limit in compression. Thislimit can be avoided by summing the tension in all the control wires andchecking to see that they do not exceed the compressive strength of theshaft.

Another exemplary aspect of the present disclosure is thatnon-idealities such as signal saturation may be addressed. For example,an anti-windup algorithm may be included on the integral control termsuch that the commanded electrical torque, τ_(elec), will never actuallyexceed what the electronics are capable of producing. Similarly, allcontrol terms are pre-saturated before adding them together to calculateτ_(elec) such that any one term cannot over-power the others when actingat the maximum of the available range.

Further, if a fault condition was detected when driving in tensioncontrol mode relating to the tension sensor or another aspect of thetension control mode, the system could be shut down the system or, inthe alternative, an automatic switch to position control could beimplemented. The tension control mode has the advantage of compatibilitywith the position control mode, enabling the two modes to back up theother. The catheter kinematics and solid mechanics for the modes enablesmooth transition from tension control mode into position control mode.This would allow us to keep the doctor in control of the catheter in thecase of a fault that only affects the tension sensing system withoutabruptly ending the procedure. Naturally, some care would have to betaken to avoid the potential for unintentional motion as the algorithmstransition.

Another embodiment of the present disclosure would include the abilityof a surgeon or other user to switch between the torque-based paradigmand the position-based paradigm, with one or the other serving as adefault mode. For example, the position-based paradigm could be utilizedas the default mode with the ability for a surgeon to switch to thetorque-based mode for certain procedures. The control computer couldautomatically switch between the modes based on parameters that areinput into the system or feedback during the procedure. Alternatively,the instrument driver may incorporate the desired motor torque value ina closed loop system. A variation of the present disclosure contemplatesusing both the position-based and torque-based paradigms during asurgical procedure, where, e.g., the system implements catheterarticulation in the first instance through motor displacement commandsbut the associated desired motor torque is used to ensure that thedesired deflection angle is achieved and/or maintained. Further, inanother embodiment of the present disclosure, other modes of operationcould be enabled, such as the ability of a constant force mode, in whichthe articulation force is maintained, creating a force against thetissue that is more uniform while crossing trabeculated tissue.

Another aspect of the current disclosure is to use the torque sensingcapability on the pull-wires to pull all wires at the same time andstiffen the catheter. For typical articulation of a catheter as shown inFIG. 12A, one wire is pulled by a distance x₁ and the opposing wire istypically released by a distance y1. This delta wire displacement causesthe tip of the catheter to deflect as described above. However, if bothwires are pulled equal amounts, then the tip will not deflect. Instead,the tensioned wires inside the wall of the catheter serve to stiffen thecatheter. One of catheter design challenge is that there are times whena catheter needs to be stiff to provide stability for deliveringtherapeutic devices through it and there are times when it needs to beflexible to navigate through tortuous vessels. Many conventionalcatheters attempt to overcome this issue by designing catheters withvariable stiffness—that is to say the distal end is manufactured withless stiff materials than the proximal end. Tension sensing on thecatheter pull wires allows dynamic variable stiffness. In other words,the catheter can be manufactured with less stiff materials and thedoctor has the ability to increase tension on all wires and stiffen thecatheter when required. It should be understood that this dynamicvariable stiffness can be applied in any catheter configuration and withany given catheter tip articulation. For example, if one pull-wire ispulled by 1N more than another, then the catheter tip may be deflected30°. The dynamic variable stiffness algorithm would now apply anadditional 2N to each of the 2 wires to increase the stiffness. Thedelta between the two wires is still 1N so the tip does not changeangle.

Operator workstation 112 may include a computer or a computer readablestorage medium implementing the operation of instrument driver 108. Ingeneral, computing systems and/or devices, such as the processor and theuser input device, may employ any of a number of computer operatingsystems, including, but by no means limited to, versions and/orvarieties of the Microsoft Windows® operating system, the Unix operatingsystem (e.g., the Solaris® operating system distributed by OracleCorporation of Redwood Shores, Calif.), the AIX UNIX operating systemdistributed by International Business Machines of Armonk, N.Y., theLinux operating system, the Mac OS X and iOS operating systemsdistributed by Apple Inc. of Cupertino, Calif., and the Androidoperating system developed by the Open Handset Alliance.

Computing devices generally include computer-executable instructions,where the instructions may be executable by one or more computingdevices such as those listed above. Computer-executable instructions maybe compiled or interpreted from computer programs created using avariety of programming languages and/or technologies, including, withoutlimitation, and either alone or in combination, Java™, C, C++, VisualBasic, Java Script, Perl, etc. In general, a processor (e.g., amicroprocessor) receives instructions, e.g., from a memory, acomputer-readable medium, etc., and executes these instructions, therebyperforming one or more processes, including one or more of the processesdescribed herein. Such instructions and other data may be stored andtransmitted using a variety of computer-readable media.

A computer-readable medium (also referred to as a processor-readablemedium) includes any non-transitory (e.g., tangible) medium thatparticipates in providing data (e.g., instructions) that may be read bya computer (e.g., by a processor of a computer). Such a medium may takemany forms, including, but not limited to, non-volatile media andvolatile media. Non-volatile media may include, for example, optical ormagnetic disks and other persistent memory. Volatile media may include,for example, dynamic random access memory (DRAM), which typicallyconstitutes a main memory. Such instructions may be transmitted by oneor more transmission media, including coaxial cables, copper wire andfiber optics, including the wires that comprise a system bus coupled toa processor of a computer. Common forms of computer-readable mediainclude, for example, a floppy disk, a flexible disk, hard disk,magnetic tape, any other magnetic medium, a CD-ROM, DVD, any otheroptical medium, punch cards, paper tape, any other physical medium withpatterns of holes, a RAM, a PROM, an EPROM, a FLASH-EEPROM, any othermemory chip or cartridge, or any other medium from which a computer canread.

Databases, data repositories or other data stores described herein mayinclude various kinds of mechanisms for storing, accessing, andretrieving various kinds of data, including a hierarchical database, aset of files in a file system, an application database in a proprietaryformat, a relational database management system (RDBMS), etc. Each suchdata store is generally included within a computing device employing acomputer operating system such as one of those mentioned above, and areaccessed via a network in any one or more of a variety of manners. Afile system may be accessible from a computer operating system, and mayinclude files stored in various formats. An RDBMS generally employs theStructured Query Language (SQL) in addition to a language for creating,storing, editing, and executing stored procedures, such as the PL/SQLlanguage mentioned above.

In some examples, system elements may be implemented ascomputer-readable instructions (e.g., software) on one or more computingdevices (e.g., servers, personal computers, etc.), stored on computerreadable media associated therewith (e.g., disks, memories, etc.). Acomputer program product may comprise such instructions stored oncomputer readable media for carrying out the functions described herein.

With regard to the processes, systems, methods, heuristics, etc.described herein, it should be understood that, although the steps ofsuch processes, etc. have been described as occurring according to acertain ordered sequence, such processes could be practiced with thedescribed steps performed in an order other than the order describedherein. It further should be understood that certain steps could beperformed simultaneously, that other steps could be added, or thatcertain steps described herein could be omitted. In other words, thedescriptions of processes herein are provided for the purpose ofillustrating certain embodiments, and should in no way be construed soas to limit the claims.

Accordingly, it is to be understood that the above description isintended to be illustrative and not restrictive. Many embodiments andapplications other than the examples provided would be apparent uponreading the above description. The scope should be determined, not withreference to the above description, but should instead be determinedwith reference to the appended claims, along with the full scope ofequivalents to which such claims are entitled. It is anticipated andintended that future developments will occur in the technologiesdiscussed herein, and that the disclosed systems and methods will beincorporated into such future embodiments. In sum, it should beunderstood that the application is capable of modification andvariation.

All terms used in the claims are intended to be given their broadestreasonable constructions and their ordinary meanings as understood bythose knowledgeable in the technologies described herein unless anexplicit indication to the contrary in made herein. In particular, useof the singular articles such as “a,” “the,” “said,” etc. should be readto recite one or more of the indicated elements unless a claim recitesan explicit limitation to the contrary.

What is claimed is:
 1. A system comprising: an elongated flexiblemedical instrument comprising an elongate member coupled to a distalportion of the medical instrument; an instrument driver coupled to aproximal end of the elongate member, the instrument driver comprising: arotatable body configured to transmit torque from a motor, and a forcesensor configured to generate an actual force signal based on an actualforce; and a control system in communication with the instrument driver,the control system configured to: generate a command force signal inresponse to a user command from a user input device, provide the commandforce signal to the instrument driver to control movement of the distalportion of the medical instrument into a first position, receive theactual force signal from the instrument driver, and maintain the firstposition of the distal portion of the medical instrument via adjustingthe command force signal provided to the instrument driver based on theactual force signal received from the instrument driver.
 2. The systemof claim 1, wherein the force sensor is further configured to provide adirect measurement of the actual force on the elongate member.
 3. Thesystem of claim 1, wherein the force sensor is further configured tomeasure a resistivity of the elongate member as a strain gauge.
 4. Thesystem of claim 1, wherein the force sensor comprises a torque sensor.5. The system of claim 1, wherein the control system is furtherconfigured to detect a fault in the force sensor by comparing anexpected force on the motor based on a current conveyed to the motor toan actual force detected by the force sensor.
 6. The system of claim 1,wherein the rotatable body comprises a drive shaft.
 7. The system ofclaim 1, wherein the motor is part of a motor servo.
 8. The system ofclaim 1, wherein the rotatable body protrudes from a face of theinstrument driver.
 9. The system of claim 1, wherein a torque sensor isinstalled on the rotatable body.
 10. The system of claim 1, wherein therotatable body is coupled to a proximal end of the elongate member. 11.The system of claim 10, wherein the elongate member is configured totransmit a tension to the distal portion of the medical instrument tocause the medical instrument to articulate in response to a rotation ofthe rotatable body.
 12. The system of claim 1, wherein the controlsystem is configured to monitor a stress on the elongate member based onthe actual force signal.
 13. The system of claim 1, wherein the medicalinstrument comprises at least one of a catheter and an endoscope.